Post-mortem analyses of brains from patients with Parkinson disease who received fetal mesencephalic transplants show that α-synuclein-containing (α-syn-containing) Lewy bodies gradually appear in grafted neurons. Here, we explored whether intercellular transfer of α-syn from host to graft, followed by seeding of α-syn aggregation in recipient neurons, can contribute to this phenomenon. We assessed α-syn cell-to-cell transfer using microscopy, flow cytometry, and high-content screening in several coculture model systems. Coculturing cells engineered to express either GFP-or DsRed-tagged α-syn resulted in a gradual increase in double-labeled cells. Importantly, α-syn-GFP derived from 1 neuroblastoma cell line localized to red fluorescent aggregates in other cells expressing DsRed-α-syn, suggesting a seeding effect of transmitted α-syn. Extracellular α-syn was taken up by cells through endocytosis and interacted with intracellular α-syn. Next, following intracortical injection of recombinant α-syn in rats, we found neuronal uptake was attenuated by coinjection of an endocytosis inhibitor. Finally, we demonstrated in vivo transfer of α-syn between host cells and grafted dopaminergic neurons in mice overexpressing human α-syn. In summary, intercellularly transferred α-syn interacts with cytoplasmic α-syn and can propagate α-syn pathology. These results suggest that α-syn propagation is a key element in the progression of Parkinson disease pathology.

The origin of a-synuclein (a-syn)-positive glial cytoplasmic inclusions found in oligodendrocytes in multiple system atrophy (MSA) is enigmatic, given the fact that oligodendrocytes do not express a-syn mRNA. Recently, neuron-to-neuron transfer of a-syn was suggested to contribute to the pathogenesis of Parkinson's disease. In this study, we explored whether a similar transfer of a-syn might occur from neurons to oligodendrocytes, which conceivably could explain how glial cytoplasmic inclusions are formed. We studied oligodendrocytes in vitro and in vivo and examined their ability to take up different a-syn assemblies. First, we treated oligodendrocytes with monomeric, oligomeric, and fibrillar forms of a-syn proteins and investigated whether a-syn uptake is dynamin-dependent. Second, we injected the same a-syn species into the mouse cortex to assess their uptake in vivo. Finally, we monitored the presence of human a-syn within rat oligodendroglial cells grafted in the striatum of hosts displaying Adeno-Associated Virus-mediated overexpression of human a-syn in the nigro-striatal pathway. Here, we show that oligodendrocytes take up recombinant a-syn monomers, oligomers and, to a lesser extent, fibrils in vitro in a concentration and time-dependent manner, and that this process is inhibited by dynasore. Further, we demonstrate in our injection model that oligodendrocytes also internalize a-syn in vivo. Finally, we provide the first direct evidence that a-syn can transfer to grafted oligodendroglial cells from host rat brain neurons overexpressing human a-syn. Our findings support the hypothesis of a neuron-to-oligodendrocyte transfer of a-syn, a mechanism that may play a crucial role in the progression and pathogenesis of MSA.

Several people with Parkinson’s disease have been treated with intrastriatal grafts of fetal dopaminergic neurons. Following autopsy, 10–22 years after surgery, some of the grafted neurons contained Lewy bodies similar to those observed in the host brain. Numerous studies have attempted to explain these findings in cell and animal models. In cell culture, α-synuclein has been found to transfer from one cell to another, via mechanisms that include exosomal transport and endocytosis, and in certain cases seed aggregation in the recipient cell. In animal models, transfer of α-synuclein from host brain cells to grafted neurons has been shown, but the reported frequency of the event has been relatively low and little is known about the underlying mechanisms as well as the fate of the transferred α-synuclein. We now demonstrate frequent transfer of α-synuclein from a rat brain engineered to overexpress human α-synuclein to grafted dopaminergic neurons. Further, we show that this model can be used to explore mechanisms underlying cell-to-cell transfer of α-synuclein. Thus, we present evidence both for the involvement of endocytosis in α-synuclein uptake in viv o, and for seeding of aggregation of endogenous α-synuclein in the recipient neuron by the transferred α-synuclein. Finally, we show that, at least in a subset of the studied cells, the transmitted α-synuclein is sensitive to proteinase K. Our new model system could be used to test compounds that inhibit cell-to-cell transfer of α-synuclein and therefore might retard progression of Parkinson neuropathology.

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Classically, Parkinson's disease (PD) is linked to dopamine neuron death in the substantia nigra pars compacta. Intracytoplasmic protein inclusions named Lewy bodies, and corresponding Lewy neurites found in neuronal processes, are also key features of the degenerative process in the substantia nigra. The molecular mechanisms by which substantia nigra dopamine neurons die and whether the Lewy pathology is directly involved in the cell death pathway are open questions. More recently, it has become apparent that Lewy pathology gradually involves greater parts of the PD brain and is widespread in late stages. In this review, we first discuss the role of misfolded a-synuclein protein, which is the main constituent of Lewy bodies, in the pathogenesis of PD. We then describe recent evidence that a-synuclein might transfer between cells in PD brains. We discuss in detail the possible molecular mechanisms underlying the proposed propagation and the likely consequences for cells that take up a-synuclein. Finally, we focus on aspects of the pathogenic process that could be targeted with new pharmaceutical therapies or used to develop biomarkers for early PD detection. Multiple hypotheses exist to help explain dopamine neuron cell death and Lewy body formation observed in Parkinson's disease (PD). Mutations of the main proteinaceous constituent of Lewy bodies, a-synuclein (a-syn), lead to dominant, familial disease forms. [1][2][3][4][5] More recently, genome-wide association studies (GWASs) identified variants of the a-synuclein gene (SNCA) gene, encoding a-syn protein, that are coupled to increased PD susceptibility, thus clearly linking this protein to idiopathic PD. 6-8 Furthermore, overexpressed and/or misfolded a-syn is pathogenic to cells while a-syn can be secreted from cells, enter other cells, and seed small intracellular aggregates, demonstrating a connection between a-syn and pathogenic mechanisms of PD. [9][10][11][12][13][14][15][16] In parallel, a much-discussed hypothesis by Braak and colleagues 17 states that a pathogenic agent, introduced via ingestion and/or inhalation, may transfer from the entry site along known long, unmyelinated axons to basal brain areas and eventually to brain stem and cortical regions. Although a-syn is unlikely to be this initial pathogen, it might be the initial target of the unknown agent. If a-syn is misfolded because of the action of the unknown agent, it might contribute to the spreading of pathology by moving from one cell to another and triggering misfolding in the recipient cells. If so, it might explain the surprising presence of Lewy bodies in the young neural grafts of transplanted PD patients observed more than a decade after surgery. [18][19][20][21] In this review, we discuss possible mechanisms by which a-syn could spread between cells and have deadly consequences by describing the molecular evidence for a-syn cellular exit, transit to other cells, uptake by cells, intracellular aggregation, and responses to a-syn accumulation.The Relationship Between a-Syn and PD ...